A variety of electrochemical cells falls within a category of cells often referred to as solid polymer electrolyte (“SPE”) cells. An SPE cell typically employs a membrane of a cation exchange polymer that serves as a physical separator between the anode and cathode while also serving as an electrolyte. SPE cells can be operated as electrolytic cells for the production of electrochemical products or they may be operated as fuel cells.
Fuel cells are electrochemical cells that convert reactants, namely fuel and oxidant fluid streams, to generate electric power and reaction products. A broad range of reactants can be used in fuel cells and such reactants may be delivered in gaseous or liquid streams. For example, the fuel stream may be substantially pure hydrogen gas, a gaseous hydrogen containing reformate stream, or an aqueous alcohol, for example methanol in a direct methanol fuel cell (DMFC). The oxidant may, for example, be substantially pure oxygen or a dilute oxygen stream such as air.
In SPE fuel cells, the solid polymer electrolyte membrane is typically perfluorinated sulfonic acid polymer membrane in acid form. Such fuel cells are often referred to as proton exchange membrane (“PEM”) fuel cells. The membrane is disposed between and in contact with the anode and the cathode. Electrocatalysts in the anode and the cathode typically induce the desired electrochemical reactions and may be, for example, a metal black, an alloy or a metal catalyst supported on a substrate, e.g., platinum on carbon. SPE fuel cells typically also comprise a porous, electrically conductive sheet material that is in electrical contact with each of the electrodes, and permit diffusion of the reactants to the electrodes. In fuel cells that employ gaseous reactants, this porous, conductive sheet material is sometimes referred to as a gas diffusion backing and is suitably provided by a carbon fiber paper or carbon cloth. An assembly including the membrane, anode and cathode, and gas diffusion backings for each electrode, is sometimes referred to as a membrane electrode assembly (“MEA”). Bipolar plates, made of a conductive material and providing flow fields for the reactants, are placed between a number of adjacent MEAs. A number of MEAs and bipolar plates are assembled in this manner to provide a fuel cell stack.
In fabricating unitized MEAs, multilayer MEAs may be sealed using a fluid impermeable polymer seal. Several techniques may be used to form these seals, including compression molding and injection molding. With injection molding, the sealing polymer that is used as the sealant material is applied in liquid or slurry form and this is associated with its own disadvantages. In injection molding, the sealing polymer sometimes does not flow onto both sides of the membrane, and the relatively high pressures and flow velocities may damage the gas diffusion backings. Balancing the pressures on all edges of the gas diffusion backings may be difficult. Another disadvantage of injection molding is the difficulty of maintaining the position of the components of the MEA in the mold. Clamping force on the components must be great enough to impede motion due to the injection pressure and may damage the fibers in the gas diffusion backing, creating debris and possible shorting of the MEA if the debris punctures the membrane. Since compression molding does not involve high-pressure gradients and flow velocities, it does not generally have these problems.
A need exists for a mold useful in compression molding, wherein membranes that are substantially dimensionally unstable are used, that does not result in a damaged unitized MEA because of the application of heat in the compression molding process.